ATHENS, Greece (AP) — Police on the Greek island of Lesbos began placing migrants and refugees on boats bound to Turkey, the first to be deported under a European Union plan to limit the amount of migration to Europe.

Under heavy security, the first 135 migrants were being escorted onto two small ferries as dawn broke Monday by officers from the EU border protection agency, Frontex, to nearby ports on the Turkish coast, under the plan which has been strongly criticized by human rights groups.

About 4,000 migrants and refugees are being detained on Greek islands since the agreement came into effect March 20.

On the nearby island of Chios, riot police clashed with local residents hours earlier during a protest against deportations planned there.

"This is the first day of a very difficult time for refugee rights. Despite the serious legal gaps and lack of adequate protection in Turkey, the EU is forging ahead with a dangerous deal," Giorgos Kosmopoulos, head of Amnesty International in Greece, told the Associated Press from Lesbos.

"Turkey is not a safe third country for refugees. The EU and Greek authorities know this and have no excuse."

The operation was supervised by a lieutenant general of the Greek police, which gave no immediate details of the nationalities of migrants being deported.

The deportations were due to start with migrants who did not apply for asylum or had their applications declared inadmissible.

"Even if this first group is not refugees, what we are seeing here is symbolic kick off of what might be a very dangerous practice of returns to Turkey," Kosmopoulos said.

A total of 50,000 migrants and refugees are stranded in Greece following EU and Balkan border closures, but only those who arrived after March 20 will be detained for deportation.